mempalace: drop the stage ENV pin, fix the shared-server compose

The feeder now defaults to <palace-root>/pi-stage upstream, so pinning
MEMPALACE_PI_STAGE into ~/.pi here is unnecessary -- and was actively wrong. It
created a second convention that could still diverge from the palace: keep the
devbox-palace volume, drop devbox-pi-config, and a scoped `mempalace sync`
prunes every conversation drawer, because dedup keys on the staged path. Both
the ENV and the entrypoint export are gone; a comment explains why adding one
back re-introduces the split it was meant to fix.

docker-compose.mempalace.yml was broken on mempalace 3.6.0 in both directions:
  - `--host 0.0.0.0` with no token in the environment makes the server refuse
    to start, crash-looping under `restart: unless-stopped`.
  - Supply a token and the healthcheck's unauthenticated `tools/list` POST 401s,
    marking a perfectly healthy server unhealthy forever.
Now the token is required via ${MEMPALACE_REMOTE_TOKEN:?...} so it fails fast at
`docker compose up` with a readable message, and the healthcheck probes the
deliberately token-free /healthz. The "no authentication of its own" security
note has been stale since 3.6.0 and is replaced with the actual posture
(bearer token + Host pin + Origin allowlist), including why browser-shaped auth
must not be put in front of it.

Dockerfile.base: mempalace-pi-session symlinked onto PATH, with a build-time
`--help` check so a broken feeder fails the image build rather than the first
session.

smoke-test: assert the stage resolves beside the palace (default, and following
$MEMPALACE_PALACE_PATH) instead of asserting the removed ENV pin. The two
behavioural guards -- a synthetic session that must be captured, an abandoned
one that must not be -- are unchanged.

.env.example: recommend `mempalace serve` on the docker0 gateway rather than
`mempalace-mcp --transport http --host 0.0.0.0`, with the two binds to avoid.

entrypoint-user.sh

@@ -92,6 +92,56 @@ if command -v mempalace &>/dev/null && [ -d /workspace ]; then
   fi
 fi
 
+# ── MemPalace: pi transcript feeder ─────────────────────────────────
+# mempalace-toolkit ships `mempalace-pi-session`, which mines pi's own JSONL
+# session transcripts into the palace. pi's mempalace extension drives it on
+# session_shutdown and on a debounced agent_settled; this is the catch-up for
+# the one case no handler can cover — a hard kill (docker kill, OOM, host
+# reboot) runs nothing at all, so without this the previous life's transcripts
+# are never mined.
+#
+# No MEMPALACE_PI_STAGE override here on purpose: the feeder stages next to the
+# palace it feeds (<palace-root>/pi-stage), so the stage and the dedup keys
+# referencing it share one lifetime — whatever persistence the palace has, the
+# stage inherits. Pinning it elsewhere (e.g. into the ~/.pi volume) would
+# re-introduce the very split that design prevents: palace volume kept, stage
+# volume dropped, and `mempalace sync` then prunes every conversation drawer.
+#
+# Backgrounded: a cold mine can take tens of seconds and must never delay the
+# shell. Contention with a live session is handled by the tool itself (it exits
+# 0 and lets the palace holder do the mine).
+
+# Self-heal onto PATH for images whose base predates the toolkit symlink.
+# ~/.local/bin is already ahead of /usr/local/bin on PATH (Dockerfile.base sets
+# it in ENV PATH) and is writable by this (non-root) user, unlike /usr/local/bin.
+if [ -x /opt/mempalace-toolkit/bin/mempalace-pi-session ] && \
+   ! command -v mempalace-pi-session >/dev/null 2>&1; then
+  mkdir -p "$HOME/.local/bin"
+  ln -sf /opt/mempalace-toolkit/bin/mempalace-pi-session "$HOME/.local/bin/mempalace-pi-session"
+fi
+
+# Resolve the feeder explicitly rather than trusting PATH: this runs before any
+# login shell, and a silently-skipped catch-up is exactly the failure we are
+# here to prevent.
+MEMPALACE_FEEDER=""
+if command -v mempalace-pi-session >/dev/null 2>&1; then
+  MEMPALACE_FEEDER="mempalace-pi-session"
+elif [ -x /opt/mempalace-toolkit/bin/mempalace-pi-session ]; then
+  MEMPALACE_FEEDER="/opt/mempalace-toolkit/bin/mempalace-pi-session"
+fi
+
+if [ "${MEMPALACE_FEED:-1}" != "0" ] && [ -n "$MEMPALACE_FEEDER" ]; then
+  if [ -n "${MEMPALACE_REMOTE_URL:-}" ] && [ -z "${MEMPALACE_PI_SSH_TARGET:-}" ]; then
+    : # remote palace but no inbox configured — nothing we can ship to; skip quietly
+  else
+    mkdir -p "$HOME/.pi/agent"
+    (
+      "$MEMPALACE_FEEDER" --reason container-start \
+        >"$HOME/.pi/agent/mempalace-catchup.log" 2>&1 || true
+    ) &
+  fi
+fi
+
 # ── Git config defaults ──────────────────────────────────────────────
 if [ -n "${GIT_USER_NAME:-}" ] && ! git config --global user.name &>/dev/null; then
   git config --global user.name "$GIT_USER_NAME"